The pilot reported that he visually checked both fuel tanks were full prior to departure. When established in cruise he noticed that both fuel quantity indicators were now showing about half full, and slowly decreasing. A check was made for obvious leaks, as the pilot thought he may have left the tank caps off. However, none were observed. As the terrain he intended to fly over would be unsuitable for a forced landing the pilot elected to immediately land in a nearby field. The aircraft collided with a fence during the landing roll, damaging the right wing strut. After landing, both fuel tank caps were found locked and the fuel tanks were found to be almost full. It was later determined that the alternator was not charging, and the battery had subsequently discharged in flight. As a result, the depleting battery voltage caused the fuel contents indications to decrease in flight.